1. Field of the Invention
This invention is directed to a method of using textured molds, more particularly to using molds which cause a rolling-over during flow of the plastic being molded so that bubbles are carried to the back non-visable side of the sheet being molded.
2. Description of the Prior Art
In the molding of viscous plastic material to form sheets used as body panels and the like, it has become standard practice to polish dies to the highest gloss obtainable. The high polish was imparted to minimize surface irregularities. The polished die was an out growth of the development of SMC that more accurately reproduced the exact surface of the mold. Unfortunately, the highly polished dies did not accomplish the desired end result. Sheets produced using the highly polished dies still contained significant porosity, noticeable waves and sinks, fiber patterns visable on the surface, orange peel on the finished part and visable flow lines.
The material most commonly used in the compression molding of body panels has been low profile sheet molding compound (SMC). SMC is a compound consisting of polyester resin, fillers, catalysts, chopped glass strands, release agents and a low profile additive that expands during the curing reaction.
SMC has been described in U.S. Pat. Nos. 3,701,748; 3,772,241; 3,674,893, 3,577,478, 3,548,030 and 3,466,259. SMC has gained widespread use in the automotive industry as a molding compound for exterior vehicle parts due to (a) low cost, (b) ability to form large parts, and (c) ability to mold in ribs and bosses for fastening.
A persistent problem, however, has been apparent porosity and fiber prominence on the surface. These defects show up after the first application of primer and result in extensive reworking and repainting of the part. It has become a prior art standard practice in tooling for SMC to polish dies to the highest gloss obtainable to obtain an absolutely smooth flat surface.
The reason that the automotive industry has switched to body panels made from plastic sheet recently on a massive scale, has been to reduce weight, and thus, improve fuel efficiency. One benefit to the consumer was body panels that did not rust through in a year or two in northern climates where roads are salted. Other benefits included elimination of rattling noises generated by metal-to-metal contact. One difficulty with the plastic body panels has been the problem of producing a surface free of visable imperfections without a subsequent sanding operation.
To produce such a surface on an automotive panel it has been necessary, in the past, to extensively smooth the surface of panels having imperfections with sandpaper. This generates dust, often requiring the laborers doing the job to wear respirator masks. This is quite uncomfortable on hot days near the hot curing presses.
Before the development of the present invention, raised areas were welded onto the vertical backside of a grill opening panel mold and the panels produced were sold more than one year prior to the present application date.